1. Field of the Invention
An aspect of the present invention relates to a recording/reproducing method and recording/reproducing apparatus which uses an information storage medium by adaptively preparing a standard for distinguishing bit 1 from bit 0 of data read from a holographic information storage medium, and a holographic information storage medium for use with the apparatus.
2. Description of the Related Art
In optical holography, data is not stored on the surface of a recording medium but, rather, in a volume thereof. A signal beam interferes with a reference beam within the recording medium, generating a plurality of interference gratings referred to as a data page. The interference gratings overlap with each other by changing the optical characteristics of the reference beam. This process is referred to as multiplexing. When data is read from the recording medium, a single reference beam is irradiated onto the recording medium under the same conditions as used during the data recording, generating a diffraction beam indicating the stored data page. The diffraction beam is detected by a detection array, which extracts a plurality of stored data bits from a measured intensity pattern. The data page contains the data bits or pixels. When the data pages overlap in the volume of the recording medium, data storage capacity is increased.
Recording in a holographic information storage medium is carried out as a result of interference between a signal beam and a reference beam. The signal beam for recording is produced by a spatial light modulator in the form of a page comprised of a plurality of pixels. The signal beam is transmitted into an optical system and interferes with the reference beam in the holographic information storage medium. An interference pattern produced by the interference is recorded in the holographic information storage medium. Reproduction is carried out by diffraction when the reference beam is radiated onto the recorded interference pattern.
A Reed-Solomon (RS) code, used as an error correction code in a conventional optical storage such as a CD, DVD, HD DVD, Blu-ray disc, or the like, performs error correction by receiving a hard value, i.e. a plurality of bits (symbols) which can each be 1 or 0. Recently, since high density storage of hundreds of GB or even several TB is required, an input signal related to error correction may be relatively very dense. Thus, the quality of the input signal may decrease. Accordingly, error correction codes based on soft iterative decoding, which corrects errors by receiving a soft value instead of a hard value of each bit, such as an LDPC code or a turbo code, are popular due to their enhanced error correction characteristics. According to soft interactive decoding, an input signal of an error correction code is a soft value, not a hard value, such as 1 or 0. This is called soft-in soft-out (SISO). A SISO apparatus outputs a soft value by receiving data reproduced from an information storage medium or transmitted from a transmitter. The soft value is generally output in a log likelihood rate (LLR), in which a natural log value is applied to a likelihood rate of each bit 1 and bit 0.
In order to obtain a likelihood of bit 1 and bit 0 of an input soft value for a SISO, there should be some kind of statistical standard or random standard. Such a standard of distinguishing bit 1 from bit 0 can be provided by analyzing the distribution of soft values of bit 1 and bit 0 by recording and reproducing a known bit pattern. However, the accuracy of such a standard is directly related to the signal processing capacity. Thus, if the variation of the distribution of the soft values of bit 1 and bit 0 is too large, the accuracy of the standard is low, and accordingly, the signal processing capacity deteriorates.
Generally, in conventional optical storage technologies, the standard of distinguishing bit 1 and bit 0 is not fixed due to an inter-symbol interference (ISI), and varies according to the recorded bit pattern. Also in a holographic storage system which records in page units, i.e. in a two dimensional array form, two dimensional inter-pixel interference (IPI) occurs, and, as described above, the standard of distinguishing bit 1 from bit 0 in the holographic storage system is also not fixed and differs according to a recorded bit pattern. In addition, as cross-talk increases, according to page multiplexing and book multiplexing, the quality of an input signal decreases. Also, the distribution of soft values of bit 1 and bit 0 has a large variation not only in each page but also in each sub-area of a page. Accordingly, an adaptive standard is needed to distinguish bit 1 from bit 0.